Input devices (e.g., a touch screen or touch pad) are designed to detect the application of an object and to determine one or more specific characteristics of or relating to the object as relating to the input device, such as the location of the object as acting on the input device, the magnitude of force applied by the object to the input device, etc. Examples of some of the different applications in which input devices may be found include computer display devices, kiosks, games, automatic teller machines, point of sale terminals, vending machines, medical devices, keypads, keyboards, and others.
Force-based input devices are configured to measure the location and magnitude of the forces applied to and transmitted by the input pad. Force-based input devices comprise one or more force sensors that are configured to measure the applied force, either directly or indirectly. Various types of force sensors can be used, including for example piezoresistive sensors and piezoelectric transducers. The force sensors can be operated with gloved fingers, bare fingers, styli, pens, pencils or any object that can apply a force to the input pad. Typically, location and magnitude of the applied force is determined by solving mechanical moment equations for which the inputs are the forces measured by the force sensors.
Determining the location and magnitude of the applied force is complicated by errors in the output from the force sensors. In particular, force sensors tend to have a baseline output even when not being touched. Baseline error is drift in the quiescent output of the sensors, for example due to environmental effects. For example, stresses on the force sensors due to the weight of the touch surface and/or preload forces that are applied during manufacture can result in non-zero output from the force sensors. Moreover, baseline output tends drift with time due to aging, temperature, and other factors. In particular, the determination of touch location can be extremely sensitive to baseline errors. This is because baseline errors can result in a constant bias in the formulas used to calculate the touch location. For example, baseline errors which appear in denominator terms tend to cause error in scale that varies with the applied force.
It is therefore helpful to determine the baseline output of a force sensor so that this error source can be compensated. Various ways are known for determining the baseline of a force sensor. In general, baseline determination is performed during periods of no touch activity. For example, baseline determination can include averaging the sensor output during periods of no touch activity. Distinguishing between periods of touch activity and no-touch activity can, however, be difficult. For example, light touches are difficult to distinguish from baseline variations. Additionally, piezoelectric sensors exhibit a dynamic baseline shift that is a function of the applied stimulus. Thus, touching the panel causes the baseline to change, usually in a predictable manner.